Interpretive Summary: There is increasing competition for water resources between agriculture and other industries; therefore genetic and cultural techniques need to be identified that increase the efficiency of plant production without increasing water use, termed water use efficiency (WUE). Two unpruned willow leaf and two unpruned standard leaf peach selections were evaluated for physiological components related to WUE with the hypothesis that the smaller and more narrow willow leaves would allow more light to penetrate the tree canopy to reduce shading and increase WUE. The willow leaf characteristic itself did not confer improved water use efficiency but increasing light penetration into the canopy did improve WUE. The new cultivar, 'Bounty', had higher WUE than the standard cultivar 'Redhaven'. Future work in peach breeding to improve WUE and productivity must take into consideration light interception, productivity, and WUE in an integrated manner to make real progress in the efficient use of water and light in the orchard environment.

Technical Abstract:
Two unpruned willow leaf and two unpruned standard leaf peach [Prunus persica (L.) Batsch.] selections were evaluated for physiological components related to water use efficiency (WUE; carbon assimilation per unit of transpiration). The purpose of the study was to assess the value of willow leaf phenotypes to improve water use efficiency in peach and separate the environmental from the genetic components. The willow leaf characteristic itself did not confer improved water use efficiency. The interception of light was a key determinant of WUE in these genotypes and the relationship of WUE with intercepted photosynthetically active radiation (PAR) by the entire canopy indicated a significant negative correlation. Internal shading of the tree by excessive leaf area reduced WUE and canopies that intercept more than 60% of the PAR have reduced WUE. While WUE is improved by reducing the amount of PAR interception of the canopy, productivity is reduced. Neither of the willow leaf genotypes had a significant correlation of WUE with yield (leave and fruit weight); however, the standard leaf type cultivars, 'Bounty' and 'Redhaven', had significantly different regressions that indicate greater productivity in 'Bounty' for a given level of WUE. 'Redhaven' was the least productive cultivar; 'Bounty' was the most productive and the two willow leaf genotypes were intermediate in the relationship of intercepted PAR with yield. Therefore, genetic differences in peach growth types can be selected for both increased WUE as well as increased productivity. Future work in peach breeding to improve WUE and productivity must take into consideration light interception, productivity, and WUE in an integrated manner to make real progress in the efficient use of water and light in the orchard environment, in addition to identifying the mechanisms that are responsible.